Awesome question! I think the subject title is a bit misleading though.
I took it as "Easiest Armies/Enemies to beat on?" but then I read the body, and you seem to be actually asking "Toughest Adversaries' or 'problem Units of the enemy).
So, I'll throw in my cents/ shillngs worth on both. lol
EASIEST army for me seems to be Orcs, but perhaps that's because I run with the Farsight Enclaves. Once, out of five matches, did the Greenskin edge me out, and it's because I got a little too cocky with Mr. Bright Sword against an Armored Mega Knob from Hell! Other than that, the Tau have range, movement, and weapons that Ignore Cover on demand, so the Boyz are usually a Milk Run.
As far as Units that you need to always be prepared for, I believe the Imperial Knight
, in it's various forms, have proven to be a tough nut for the Tau to crack, but not impossible. You just need to be tactically aware of the best options to beat them. There are several threads on here that dive into Anti-Knight Strategy, but the most comprehensive, IMHO, is this one
Another enemy unit to watch out for is the Chaos/Demon Heldrake
, which can really be quite nasty
. A bit tough to kill, even for an army with plentiful AA options, Heldrakes can decimate Tau Fire Warriors, Stealth Teams, Pathfinders, and ESPECIALLY Kroot with ease. I usually make bringing down the "Hell Turkey" a priority.
Personally, I find Psykers
to be either annoying or incredibly damaging. Since Tau have no Psyker ability and only One, rare Item of Anti-Psyker defense, if they catch you in a tough spot, or unprepared, they WILL hurt you. Psychic Shriek is one ability I hate the most, as it can easily wipe out a squad of Infantry (even Crisis Suits if you're unlucky) without much option to save.
Lastly (for now) how about Swooping Hawks
? Do you guys encounter this? The Eldar player in my Meta has this trick he likes to do. He gets two squads of these guys, in reserve, and when they come on, they land a Pie-Plate that Ignores cover on any Fire Warriors he sees, obliterating most, if not all of them, with that Eldar Grenade Strike thingy (which punches RIGHT THROUGH Fire warrior armor), and then, as soon as he is able to (next round or the one after I think), he lifts them back off the board into Ongoing Reserves, to repeat the attack the following turn. For the love of the Greater Good, I hate that.